The goal of a sampled data system is to accurately acquire an analog signal. The required sample rate of the sampling system is dictated by the bandwidth of the analog signal to be acquired. The sample rate of the sampling system is usually at least twice that of the highest frequency being digitized. This is often called the “Nyquist rate”
Often this sample rate parameter cannot be met with a single analog-to-digital converter (or ADC), which is one type of digitizing element commonly used in this application. For this reason, a technique called interleaving is often employed. Interleaving is a technique whereby multiple digitizing elements sample the analog signal offset in time. For example, in a system having a two ADCs, the first ADC samples the signal, then the second ADC samples the signal, then the first and so on. The digital output of the ADCs may then be multiplexed or otherwise combined to yield a composite digital corresponding to the analog input signal. Use of interleaving accordingly eases the speed requirements of each of the individual ADCs.
Use of interleaving in digital oscilloscopes may accordingly provide the significant advantage of increasing the effective bandwidth of the oscilloscope. With a given set of ADCs, a substantially higher sample rate may be achieved with the use of interleaving. Increasing the sample rate correspondingly increases the maximum frequency that may be sampled by the system, which is commonly called the “bandwidth” of the oscilloscope. The term bandwidth actually refers to a frequency range rather than an upper limit. The lower end of the range is generally understood to be around 0 Hz for an oscilloscope, so the bandwidth of an oscilloscope generally corresponds to the maximum frequency that can be sampled by the system. Thus, a two-fold increase in sample rate generally yields around a two-fold increase in oscilloscope bandwidth.
However, use of interleaving usually requires matching of the timing relationship, gain, and offset of each digitizing element. When digitizers are mismatched in these characteristics the accuracy of the digitized waveform is compromised.
The quality of sampled data systems may be measured by a variety of specifications. Applicable specifications include spurious free dynamic range (SFDR), signal to noise and distortion ratio (SINAD), and effective number of bits (ENOB). SFDR is the size of the largest spurious tone (called a spur) relative to the size of the desired tone acquired (usually in dB). SINAD is the power of the desired tone acquired relative to the power of all noise and distortion components. ENOB is directly related to SINAD through a simple equation and decreases by one for each 6 dB reduction in SINAD.
One symptom of mismatched digitizers is error signal generation. One type of error signal is an artifact signal created by digital signal processing techniques. One type of artifact signal is a spurious tone. When multiple digitizers work in an interleaved configuration to digitize a waveform and a single tone is applied to the system, multiple tones result. The frequency location of the spurious tones is determined by the input frequency and the number of digitizers employed. The magnitude and phase of the spurious tones is determined by the input frequency magnitude and phase, as well as the response characteristics of the individual digitizers, including the response characteristics of the various signal paths leading to each digitizing element. These spurious tones serve to degrade the quality of the digitizing system, as measured with the aforementioned specifications.
A goal in the design of an interleaved system is to match the digitizer characteristics thereby reducing the size of the spurs. This goal can be more readily achieved when all of the digitizers utilized are housed in the same chip, thereby matching in construction and thermal characteristics and have very short paths from chip input to individual digitizer input. Attaining acceptable digitizer matching is significantly more difficult where digitizers are located on separate chips, manufactured in different lots and have long paths from the point that the analog signal is split to the chip inputs. Often, this causes the digitizer response characteristics to be mismatched in a frequency dependent manner.